As is discussed in the applicant's copending International Patent Application, filed on the same date as the present application and entitled "Add/drop Multiplexer", the content of which is incorporated herein by reference, it is possible to construct a branching element for a fiber optic component which is essentially passive. An schematic example is shown in FIG. 1A, comprising a branching element 105 having a first branch 101 for carrying signals to and from a first part of a fiber trunk, a second branch 102 for carrying signals to and from a second part of the fiber trunk, and a third branch 103 for carrying signals to and from a spur station. Although each of the "branches" 101,102,103 is shown here as a single fiber, this is only for ease of illustration: it is possible for each of branches 101, 102, 103 to comprise a plurality of fibers. Branches 101, 102 generally will comprise two or more fibers, one or more to carry traffic in one direction and one or more to carry traffic in the opposite direction. Signals at specific carrier wavelengths are routed by the system so that they are directed out of the branching element according to their carrier wavelength. In an exemplary case, signals arriving on branch 101 are allowed to pass out to branch 102, except at carrier wavelength .lambda..sub.1, when they are diverted to branch fiber 103. New signals .lambda..sub.1 ' at the same carrier wavelength are added to the branching element from the spur station along branch 103 and are passed out of the branching element along branch 102. Similarly, signals entering the branching element along branch 102 pass out on to branch 101, except at a different carrier wavelength .lambda..sub.2, at which signals are dropped to the spur station along branch 103, and replaced with other signals .lambda..sub.2 ' at this wavelength entering the branching element along branch 103 and passing out along branch 102.
A passive branching element such as indicated above can be designed to have considerable advantages: it can employ relatively few components and can be constructed so that it should not require attention at any point during its working life. Such an element is particularly suitable to use in undersea cable networks. However, it would be desirable even in this area to be able to switch a branching element to at least a limited degree. One desirable option is shown in FIG. 1B: as well as dropping .lambda..sub.1 from trunk 101 to spur 103, .lambda..sub.3 is dropped and replaced with a new signal from spur 103 at the same carrier wavelength. Alternatively, .lambda..sub.3 could be dropped instead of .lambda..sub.1. Ability to provide such features may allow the network as a whole to be reconfigured (for example, by the addition or removal of spur stations to or from the network, or by the addition of capacity to the spur node) without any need physically to change or replace individual branching units already in place.
Further desirable options are shown in FIGS. 1C, 1D and 1E. These all relate to a break in either the trunk or the spur. All these options are of assistance in allowing traffic to still be transmitted even after such a break has occurred by routing all traffic away from the broken branch. In the FIG. 1C case of a broken spur fiber 103, all traffic to the spur is routed on to one or the other of the trunk branches 101, 102. It then passes through a preceding or a subsequent alternative branching unit and spur station (the alternative branching unit being modified, for example, by being adapted to add and drop additional carrier wavelengths by switching from a FIG. 1A configuration to a FIG. 1B configuration) and then transmitted between the fiber break spur station and the alternative spur station by means of a back-haul network (e.g. a land line) between the two stations. Traffic from the spur can follow the same route, but in the opposite direction. FIGS. 1D and 1E show arrangements which allow the rerouting of all traffic in response to a break in the trunk fiber. All signals for transmission to the broken trunk 102 in FIG. 1D are dropped down spur 103 and communicated through a back-haul network to another spur for a branching unit as shown in FIG. 1E, so that all signals to travel along the trunk fiber are routed around the fiber break.
In C. R. Giles and V. Mizrahi, IOOC-95, ThC2-1,pp 66-67, an experimental arrangement is shown including a simple add/drop multiplexer in which the add/drop wavelength can be changed. An add/drop multiplexer of this general type is shown in FIG. 15. The signal path, which links a first optical circulator 901 at which a signal may be dropped and a second optical circulator 902 at which a signal may be added, goes into a first 1.times.2 optical switch 903 and out from a second 1.times.2 optical switch 904. The two optical switches are linked by a first path with a Bragg grating 905 to reflect light at .lambda..sub.1 and also by a second path with a Bragg grating 906 to reflect light at .lambda..sub.2, with the result that the add/drop wavelength of the multiplexer can be switched between .lambda..sub.1 and .lambda..sub.2 : the connection to ports of the circulators is such that only signals of the carrier wavelengths reflected by the Bragg grating on the chosen signal path will be added or dropped. This document does not however provide or suggest a full solution to the problem of constructing rerouting mechanisms for use in branching units of a fiber optic network to achieve the functionality of FIGS. 1B to 1E.
There is thus a need to provide simple and economical switching mechanisms to achieve signal rerouting with the functionalities indicated in FIGS. 1B to 1E. Generally, there is a need to provide simple and reliable switching for branching units of a fiber optic network.
Accordingly, the invention provides a branching unit for a fiber optic network adapted to carry signals at a plurality of predetermined carrier wavelengths, comprising one or more inputs for receiving signals either from one or more trunk fibers of the network or from spur fibers for adding signals from spur stations of the network, one or more outputs for outputting signals either to one or more trunk fibers of the network or to spur fibers for dropping signals to spur stations of the network, and an add/drop multiplexer and switching means to provide two or more different routings of signals between said inputs and said outputs.
In one advantageous form, said switching means is adapted to provide alternative signal routings such that signals at one or more predetermined carrier wavelengths entering the branching unit at one input are directed in said alternative signal routings to alternative outputs of the branching unit. In another advantageous form, said switching means is adapted to provide a normal signal routing and an alternative signal routing, such that in said alternative signal routing, signals are rerouted from one or more designated outputs of the branching unit to one or more other outputs of the branching unit.
In certain preferred embodiments, said switching means comprises one or more switching elements having a first state in which signals pass directly therethrough and a second state in which signals are diverted around a loop path with one or more wavelength routing components thereon. At least one of said switching elements may be provided within said add/drop multiplexer: for at least one of said switching elements the add/drop multiplexer may be provided within the loop path.
Advantageously, the switching means comprises a prerouting switch network connected between inputs and outputs of said add/drop multiplexer and said inputs and outputs of the branching unit to enable rerouting of signals away from one or more of the branching unit outputs. Preferably, said signals rerouted away from the one or more of the branching unit outputs do not pass through the add/drop multiplexer. It is preferred that said prerouting switch network comprises a plurality of 2.times.2 optical switches, especially fused fiber switches comprising a fiber optic coupler in which the switching is accomplished by bending of the coupler fibers.
The invention further provides a branching unit for a fiber optic network adapted to carry signals at a plurality of predetermined carrier wavelengths, comprising one or more inputs for receiving signals from fibers of the network, one or more outputs for outputting signals to fibers of the network, and an add/drop multiplexer to route signals between said one or more inputs and said one or more outputs and switching means comprising one or more 2.times.2 optical switches to provide at least one alternative routing of signals between said one or more inputs and said one or more outputs.
In a further aspect, the invention provides a wavelength routing element for wavelength division multiplexing in a fiber optic network, comprising a linear array of switching segments defining a signal line between an input and an output of the wavelength routing element, and further comprising an input for a control signal, wherein each switching segment comprises: means for rerouting signals at one or more predetermined carrier wavelengths, and means for switching said signal rerouting means in or out of the signal line in response to a component of the control signal relating to said switching segment, such that signals at a chosen set of predetermined carrier wavelengths are reroutable according to the components of the control signal. Advantageously, there are N switching segments in said linear array, and for the Nth switching segment the signal rerouting means comprises a fiber Bragg grating for a carrier wavelength .lambda..sub.N, so that components of the control signal are selectable to allow any carrier wavelength .lambda..sub.N, to be reflected or transmitted by the wavelength routing element. In this aspect, the invention further provides a multi-wavelength filter, and also an add/drop multiplexer, comprising such a wavelength routing element.
In a still further aspect, the invention provides a wavelength routing element for wavelength division multiplexing in a fiber optic network, comprising an input, an output and an input/output, means for selective routing of signals at predetermined wavelengths either from the input to the output or between the input/output and the input or output, and switching means for the selective signal routing means to switch the signal routing provided thereby, wherein the signal routing means is adapted such that a path for signals for routing from the input to the output irrespective of the state of the switching means is not even temporarily affected by any activation of the switching means.
Wavelength routing elements as described above may be used in add/drop multiplexers, and such add/drop multiplexers may be particularly adapted for use in branching units as indicated above.
In a further aspect, the invention provides a wavelength routing element, comprising an input, an output, and a switching element having a first state in which signals pass directly therethrough from the input to the output and a second state in which signals are diverted around a loop path with one or more wavelength routing components thereon.
The invention further provides a method of routing signals at a plurality of predetermined carrier wavelengths between stations of a fiber optic network, comprising directing said signals into branching units comprising both one or more passive add/drop multiplexers which provide a predetermined routing of signals according to carrier wavelength, and a switching network, and by operating the switching network so as to change the routing of signals within said branching units for one or more predetermined carrier wavelengths.
Such a method may be particularly adapted such that a signal path is provided within said branching units for signals for onward transmission along the fiber optic network which is not even temporarily affected by activation of any of the switching elements within one of said add/drop multiplexers.
Specific embodiments of the invention are described below, by way of example, particularly with reference to the accompanying drawings, of which: